The present invention relates to a display unit with a non-luminous screen of transmissive-type and also to a backlight unit placed behind the screen.
Liquid crystal panels are usually provided with a backlight unit composed of cold cathode fluorescent lamps (CCFL). However, there is a demand for a mercury-free backlight unit from the standpoint of environmental protection. Light emitting diodes (LEDs) are regarded as promising in this respect as a substitute for CCFLs. Application of LEDs to televisions is under meticulous research because of their ability to produce a well-balanced white color by optical color mixing from LEDs emitting red, green, and blue elementary colors.
The disadvantage of using LEDs as the light source is the necessity of supplying red LEDs, green LEDs, and blue LEDs with current independently because they differ in luminous efficiency. Moreover, LEDs vary in their semiconductor compositions depending on their emitting color and hence vary in their driving voltage and power consumption. However, it is not practical to individually drive LEDs constituting the backlight unit.
In practical use of LEDs as the light source of backlight unit, it is common practice to divide LEDs into a certain number of groups and drive each group (in which LEDs are connected in series) as a whole.
In other words, each group consists of a prescribed number of red, green, and blue LEDs connected in series, and each group is connected to a DC-DC converter power source and a PWM control unit. Thus it is possible to adjust the tint and brightness of backlight (which result from combination of red, green, and blue) by adjusting the current (through PWM control) supplied to the LEDs in each group.    Patent Document 1:    Japanese Patent Laid-open No. 2001-272938